diff --git a/tests/lib/claude-plugin-setup.test.js b/tests/lib/claude-plugin-setup.test.js
index cdda9f5f0..f3715d3c5 100644
--- a/tests/lib/claude-plugin-setup.test.js
+++ b/tests/lib/claude-plugin-setup.test.js
@@ -4,6 +4,7 @@ const assert = require('assert');
 const fs = require('fs');
 const os = require('os');
 const path = require('path');
+const { withoutShadowingClaude } = require('./helpers/windows-test-env');
 
 const repoRoot = path.join(__dirname, '..', '..');
 const fakeClaudeScript = path.join(repoRoot, 'tests', 'fixtures', 'fake-claude-plugin.js');
@@ -87,7 +88,7 @@ function withFixture(initialState, fn) {
     process.chdir(fixture.projectRoot);
     process.env.HOME = fixture.homeDir;
     process.env.USERPROFILE = fixture.homeDir;
-    process.env.PATH = `${fixture.binDir}${path.delimiter}${previous.PATH || ''}`;
+    process.env.PATH = `${fixture.binDir}${path.delimiter}${withoutShadowingClaude(previous.PATH)}`;
     process.env.CLAUDE_CONFIG_DIR = fixture.configDir;
     process.env.ECC_TEST_CLAUDE_STATE = fixture.statePath;
     process.env.ECC_TEST_CLAUDE_CALLS = fixture.callsPath;
diff --git a/tests/lib/claude-scope-migration.test.js b/tests/lib/claude-scope-migration.test.js
index 91ce940ba..c3ab6606f 100644
--- a/tests/lib/claude-scope-migration.test.js
+++ b/tests/lib/claude-scope-migration.test.js
@@ -4,6 +4,7 @@ const assert = require('assert');
 const fs = require('fs');
 const os = require('os');
 const path = require('path');
+const { withoutShadowingClaude } = require('./helpers/windows-test-env');
 
 const repoRoot = path.join(__dirname, '..', '..');
 const fakeClaudeScript = path.join(repoRoot, 'tests', 'fixtures', 'fake-claude-plugin.js');
@@ -100,7 +101,7 @@ function withFixture(initialState, fn) {
     process.chdir(fixture.projectRoot);
     process.env.HOME = fixture.homeDir;
     process.env.USERPROFILE = fixture.homeDir;
-    process.env.PATH = `${fixture.binDir}${path.delimiter}${previous.PATH || ''}`;
+    process.env.PATH = `${fixture.binDir}${path.delimiter}${withoutShadowingClaude(previous.PATH)}`;
     process.env.CLAUDE_CONFIG_DIR = fixture.configDir;
     process.env.ECC_TEST_CLAUDE_STATE = fixture.statePath;
     process.env.ECC_TEST_CLAUDE_CALLS = fixture.callsPath;
diff --git a/tests/lib/helpers/windows-test-env.js b/tests/lib/helpers/windows-test-env.js
new file mode 100644
index 000000000..9ccfeefa6
--- /dev/null
+++ b/tests/lib/helpers/windows-test-env.js
@@ -0,0 +1,38 @@
+/**
+ * Keep tests independent of tools installed on a Windows developer machine.
+ *
+ * Linux CI never sees these leaks, so they surface only as local failures.
+ */
+
+'use strict';
+
+const fs = require('fs');
+const path = require('path');
+
+// On Windows, spawning a bare `claude` resolves only .com/.exe on PATH, never
+// .cmd. A natively installed claude.exe therefore wins over a test's fake
+// claude.cmd even when the fake's directory comes first on PATH.
+const SHADOWING_CLAUDE_BINARIES = ['claude.com', 'claude.exe'];
+
+/**
+ * Drop PATH entries holding a real Claude binary that would shadow a fake
+ * `claude.cmd` launcher. A no-op off Windows, where PATH order is honoured.
+ */
+function withoutShadowingClaude(pathValue, platform = process.platform) {
+  if (platform !== 'win32') return pathValue || '';
+  return String(pathValue || '')
+    .split(path.win32.delimiter)
+    .filter(dir => dir && !SHADOWING_CLAUDE_BINARIES.some(name => fs.existsSync(path.join(dir, name))))
+    .join(path.win32.delimiter);
+}
+
+/**
+ * Name an archive relative to the directory tar runs in, with forward slashes.
+ * GNU tar, which Git for Windows puts first on PATH, reads an archive argument
+ * containing a colon (C:\...) as a host:path remote spec; bsdtar does not.
+ */
+function toRelativeTarPath(cwd, archivePath) {
+  return path.relative(cwd, archivePath).split(path.sep).join('/');
+}
+
+module.exports = { withoutShadowingClaude, toRelativeTarPath };
diff --git a/tests/lib/windows-test-env.test.js b/tests/lib/windows-test-env.test.js
new file mode 100644
index 000000000..be888271f
--- /dev/null
+++ b/tests/lib/windows-test-env.test.js
@@ -0,0 +1,62 @@
+/**
+ * Tests for tests/lib/helpers/windows-test-env.js
+ *
+ * Run with: node tests/lib/windows-test-env.test.js
+ */
+
+'use strict';
+
+const assert = require('assert');
+const fs = require('fs');
+const os = require('os');
+const path = require('path');
+
+const { test, banner, summary } = require('./helpers/mini-test-runner');
+const { withoutShadowingClaude, toRelativeTarPath } = require('./helpers/windows-test-env');
+
+banner('windows-test-env helper');
+
+let passed = 0;
+let failed = 0;
+
+const root = fs.mkdtempSync(path.join(os.tmpdir(), 'windows-test-env-'));
+const exeDir = path.join(root, 'native-install');
+const comDir = path.join(root, 'com-install');
+const cmdDir = path.join(root, 'npm-global');
+const plainDir = path.join(root, 'tools');
+for (const dir of [exeDir, comDir, cmdDir, plainDir]) fs.mkdirSync(dir);
+fs.writeFileSync(path.join(exeDir, 'claude.exe'), '');
+fs.writeFileSync(path.join(comDir, 'claude.com'), '');
+// A .cmd shim cannot shadow a fake .cmd placed earlier on PATH, so it stays.
+fs.writeFileSync(path.join(cmdDir, 'claude.cmd'), '');
+
+try {
+  if (test('drops PATH entries holding claude.exe or claude.com on Windows', () => {
+    const pathValue = [plainDir, exeDir, cmdDir, comDir].join(';');
+    assert.strictEqual(withoutShadowingClaude(pathValue, 'win32'), [plainDir, cmdDir].join(';'));
+  })) passed++; else failed++;
+
+  if (test('drops empty entries and tolerates a missing PATH on Windows', () => {
+    assert.strictEqual(withoutShadowingClaude(`;${plainDir};;`, 'win32'), plainDir);
+    assert.strictEqual(withoutShadowingClaude(undefined, 'win32'), '');
+  })) passed++; else failed++;
+
+  if (test('leaves PATH untouched off Windows', () => {
+    const pathValue = [plainDir, exeDir].join(':');
+    assert.strictEqual(withoutShadowingClaude(pathValue, 'linux'), pathValue);
+    assert.strictEqual(withoutShadowingClaude(undefined, 'darwin'), '');
+  })) passed++; else failed++;
+
+  if (test('names an archive relatively with forward slashes and no drive letter', () => {
+    const cwd = path.join(root, 'extract', 'node_modules');
+    const archive = path.join(root, 'pkg-1.0.0.tgz');
+    const relative = toRelativeTarPath(cwd, archive);
+    assert.ok(!relative.includes(':'), `tar reads a colon as host:path: ${relative}`);
+    assert.ok(!relative.includes('\\'), `expected forward slashes: ${relative}`);
+    assert.strictEqual(path.resolve(cwd, relative), archive);
+  })) passed++; else failed++;
+} finally {
+  fs.rmSync(root, { recursive: true, force: true });
+}
+
+summary(passed, failed);
diff --git a/tests/scripts/ecc-universal-bin.test.js b/tests/scripts/ecc-universal-bin.test.js
index c4336d26a..27f508e95 100644
--- a/tests/scripts/ecc-universal-bin.test.js
+++ b/tests/scripts/ecc-universal-bin.test.js
@@ -12,6 +12,7 @@ const os = require('os');
 const path = require('path');
 const { spawnSync } = require('child_process');
 const { getNpmPackEntry } = require('../lib/npm-pack-output');
+const { toRelativeTarPath } = require('../lib/helpers/windows-test-env');
 
 const repoRoot = path.join(__dirname, '..', '..');
 const packageJson = JSON.parse(
@@ -170,8 +171,8 @@ function prepareLocalPackedProject(packageManager) {
     });
   }
   fs.mkdirSync(modulesDirectory, { recursive: true });
-  run('tar', ['-xzf', fixture.archivePath, '-C', modulesDirectory], {
-    cwd: projectDirectory,
+  run('tar', ['-xzf', toRelativeTarPath(modulesDirectory, fixture.archivePath)], {
+    cwd: modulesDirectory,
     timeout: archiveExtractionTimeoutMs,
   });
   fs.renameSync(extractedDirectory, packageDirectory);
diff --git a/tests/scripts/eval-harness-package.test.js b/tests/scripts/eval-harness-package.test.js
index b29adf69d..5d413ca8c 100644
--- a/tests/scripts/eval-harness-package.test.js
+++ b/tests/scripts/eval-harness-package.test.js
@@ -8,6 +8,7 @@ const path = require('path');
 const { spawnSync } = require('child_process');
 const { getNpmPackEntry } = require('../lib/npm-pack-output');
 const { test, tempDir, cleanup, finish, runNpm } = require('../lib/eval-harness/helpers');
+const { toRelativeTarPath } = require('../lib/helpers/windows-test-env');
 
 const repo = path.resolve(__dirname, '../..');
 const work = tempDir('package smoke');
@@ -61,7 +62,7 @@ try {
     assert.ok(archive, 'packing must succeed before extracting');
     const extract = path.join(work, 'extracted');
     fs.mkdirSync(extract);
-    const unpack = command('tar', ['-xzf', archive, '-C', extract]);
+    const unpack = command('tar', ['-xzf', toRelativeTarPath(extract, archive)], { cwd: extract });
     assert.strictEqual(unpack.status, 0, unpack.error?.message || unpack.stderr);
     const installed = path.join(extract, 'package');
     assert.ok(!fs.existsSync(path.join(installed, 'node_modules')));
diff --git a/tests/scripts/harness-audit.test.js b/tests/scripts/harness-audit.test.js
index c0087475d..c7572777c 100644
--- a/tests/scripts/harness-audit.test.js
+++ b/tests/scripts/harness-audit.test.js
@@ -635,8 +635,12 @@ function runTests() {
         );
       }
 
+      // findPluginInstall also reads USERPROFILE and os.homedir() (USERPROFILE
+      // on Windows), so isolating HOME alone lets a real install win.
       const originalHome = process.env.HOME;
+      const originalUserProfile = process.env.USERPROFILE;
       process.env.HOME = homeDir;
+      process.env.USERPROFILE = homeDir;
       try {
         const found = findPluginInstall(projectRoot);
         assert.ok(found);
@@ -647,6 +651,11 @@ function runTests() {
         } else {
           process.env.HOME = originalHome;
         }
+        if (originalUserProfile === undefined) {
+          delete process.env.USERPROFILE;
+        } else {
+          process.env.USERPROFILE = originalUserProfile;
+        }
       }
     } finally {
       cleanup(homeDir);
diff --git a/tests/scripts/setup.test.js b/tests/scripts/setup.test.js
index babc6d946..715d383c2 100644
--- a/tests/scripts/setup.test.js
+++ b/tests/scripts/setup.test.js
@@ -5,6 +5,7 @@ const fs = require('fs');
 const os = require('os');
 const path = require('path');
 const { spawnSync } = require('child_process');
+const { withoutShadowingClaude } = require('../lib/helpers/windows-test-env');
 const repoRoot = path.join(__dirname, '..', '..');
 const setupScript = path.join(repoRoot, 'scripts', 'setup.js');
 const eccScript = path.join(repoRoot, 'scripts', 'ecc.js');
@@ -62,7 +63,7 @@ function runSetup(fixture, args, options = {}) {
     HOME: fixture.homeDir,
     USERPROFILE: fixture.homeDir,
     CLAUDE_CONFIG_DIR: fixture.configDir,
-    PATH: options.path || `${fixture.binDir}${path.delimiter}${process.env.PATH || ''}`,
+    PATH: options.path || `${fixture.binDir}${path.delimiter}${withoutShadowingClaude(process.env.PATH)}`,
     ECC_TEST_CLAUDE_STATE: fixture.statePath,
     ECC_TEST_CLAUDE_CALLS: fixture.callsPath,
     ...options.env,
@@ -147,7 +148,7 @@ function runInteractiveEccSetup(fixture, options = {}) {
       HOME: fixture.homeDir,
       USERPROFILE: fixture.homeDir,
       CLAUDE_CONFIG_DIR: fixture.configDir,
-      PATH: `${fixture.binDir}${path.delimiter}${process.env.PATH || ''}`,
+      PATH: `${fixture.binDir}${path.delimiter}${withoutShadowingClaude(process.env.PATH)}`,
       ECC_TEST_CLAUDE_STATE: fixture.statePath,
       ECC_TEST_CLAUDE_CALLS: fixture.callsPath,
     },